The invention relates to improved nonwoven-film laminates and to a method for the production thereof.
Being a waterproof material and, if desired, a material that is breathable at the same time, plastic films have become indispensable in a multitude of technical fields and daily life. An broad field of application relates to personal hygiene articles, such as diapers.
While known films meet the requirements for tightness, lightness and breathability to a satisfactory extent, their stability and, above all, their surface condition which is also referred to as “grip” fail to be optimal. In particular, breathable films exhibit poor tear resistance, and the use of thicker and, thus, more stable films increases the cost incurred. As regards the grip, the smooth glossy surface of plastic films is felt to be unpleasant. Particularly the internal surface of personal hygiene articles, which is sitting directly on the skin, and the external surface as well should be felt to be soft and, if possible, give a feeling similar to textile. Smooth films give the impression of clinging to the skin, even if they are breathable films.
A further problem is the development of noise, also referred to as “rustling”, which is, in particular, caused by thin films in personal hygiene articles during movements of the wearer. Such rustling should be avoided as far as possible since, otherwise, the acceptance of the personal hygiene article would be impaired.
In order to overcome the above problems, innumerable suggestions have been made for a modification of the films as such and also for the application of laminates made of films containing woven or nonwoven fabrics.
For example, DE 195 38 049 describes a method for the production of a film web which, on the one hand, exhibits improved transverse elasticity and puncture resistance and, on the other hand, improved softness and reduced rustling. Therein, an initial film web consisting of a thermoplastic polymer is heated up to the molten state and above the crystallite melting point of the polymer by means of one or more heating cylinders and is subsequently guided through a cooled nip.
Wide-spread use is also made of laminates consisting of non-woven or woven fabrics or films, since these combine the waterproofness of the film with the textile like surface of woven or nonwoven fabrics. Nonwoven fabrics are used primarily. In the most simple case, the laminates consist of a film and a nonwoven fabric, which can be combined with each other in various manners. Thermobonding, adhesive bonding and direct extrusion (cast method) are the most current methods.
In thermobonding, a stamping roller (=engraved steel roller) is used, mostly together with a smooth steel roller as a second roller, to fuse the material of the film and/or nonwoven fabric in a localized manner by means of high temperature and pressure, with the result that the two material webs are bonded to each other. The method has the disadvantage that, owing to the conditions prevailing during bonding, the film may be damaged and may, therein, lose its liquid tightness, this also being referred to as pinholing. In addition, the bonds are only localized, this having an adverse effect on the composite strength.
U.S. Pat. No. 5,837,352 discloses an example, describing laminates consisting of a film and nonwoven fabric, which may be bonded through thermobonding or ultrasonic or other methods.
Although with adhesive bonding there is achieved a bonding across the entire surface, it results in a deterioration of the breathability of breathable films. What is more, bonding agents cause additional cost and are, in part, suspected of being harmful to health. If, however, localized bonding instead of full-surface bonding is chosen in order to preserve breathability, the composite strength will suffer.
As an alternative to adhesive bonding, films and/or nonwoven layers may be provided or additives may be introduced in the film and/or nonwoven, this allowing bonding at substantially lower temperatures, if the thermobonding method is used. U.S. Pat. No. 5,695,868 describes an example, where a component referred to as bonding agent is contained in either the film or the non-woven or even in both. This component allows thermobonding below the melting point of the film and nonwoven, with the result that the breathability of the film is preserved, and bonding remains localized.
Direct extrusion is a cost-effective method for non-breathable laminates, ensuring a reliable compound strength, but causing poor softness and a high pinholing risk. If breathable laminates are desired, breathability can only be achieved in a second step through reworking of the composite. For this purpose, either fillers causing the formation of pores thereon when the laminate is stretched are contained in the film, or the laminate is provided with pores through needling.
For example, U.S. Pat. No. 5,865,926 discloses a film which is extruded on a nonwoven web and, subsequently, the composite is stretched (ring-rolled) by means of surface-textured rollers in order to make the composite breathable.
Finally, use is also made of methods where nonwoven fabrics are provided with a coating, in order to achieve the desired tightness against liquids with simultaneous permeability to water vapor. An example thereof is described in U.S. Pat. No. 5,879,341.